Benadryl to upregulate beta 2 receptors is just a myth in my opinion. There is no scientific research to show that it upregulates beta 2's and the only thing people can ever argue in its case is "it feels like its working better."
The only way to upregulate clen is to use ketotifen fumarate, the research chemical. Taking 2-3mg daily before bed will help you stay on clen for longer and keep it effective. With use of ketotifen, you can stay on 6 straight weeks provided you dose enough keto. During the 2nd week do 3mg nightly.